V-6 Fleet rentals will start to come out of service around the end of September or early October. You should be able to locate some at Carmax or Hertz Sales.
PDR (Previous Daily Rentals) are not too shabby. I have owned 2 cars that were PDRs. Just get an extended warranty if you go this route.
As for private purchase, not many of us are inclined to get rid of the 05 since it was the initial year of the redesign. Those who are getting rid of them are upgrading to GTs and will want to get every $ they can out of them.
Hard ? really. This is an oddity in the auto world at the moment. What are you shooting for when you say "go down". And what Mustang style are you seeking; V-8 or V-6?
Jenn